This patent claims the priority of U.S. Provisional Patent Application Nos. 60/488,192 filed Jul. 17, 2003 and 60/530,814 filed Dec. 18, 2003, whose entire disclosures are herein incorporated by reference.
This invention is in the field of circuits for carrying high frequency, high bandwidth; broadband signals over twisted wire pair circuits, including the twisted pair wire circuits installed and used for carrying conventional voice telephone signals.
Broadband signals are generally described as those capable of carrying high capacity digital signals, usually between provider owned equipment at a central site and customer equipment in either home or office. These signals carry up to 8 megabits per second at present and are expected to carry up to 25 megabits per second in the near future. This digital signal is used to modulate an analog carrier signal, whose spectral bandwidth is normally, depending on the depth of modulation of the digital signal into the analog signal, a sub-multiple of the digital signal's bit rate.
Within their transmission medium, these signals are essentially a series of sine waves which may be modulated in one or more of amplitude, phase and/or frequency. The transmission medium is, for the most part, made up of twisted pairs of metallic wires gathered together into cables. Each pair is laid directly into the cable, without any metallic electrical shield between it and the surrounding pairs. These pairs are known as Unprotected Twisted Pairs (UTP). Each cable may contain any number of UTPs from 1 up to several hundred, although normal practice restricts the upper limit to 100 pairs.
Since such cables were originally designed, built and installed to carry voice frequency signals from a central telephone exchange to subscriber telephone instruments, they are not well suited to the carriage of broadband signals. Broadband signals will suffer from attenuation and the imposition of noise into the wanted signal.
However, even with severe attenuation of the wanted signal, as long as the signal to noise ratio remains sufficiently high, reception equipment will be able to extract the wanted digital signal. As the signal to noise ratio falls below an acceptable level, the number of received errors increases until the digital signal is essentially unrecoverable. It can be seen from this that noise is the limiting factor in the transmission of broadband digital signals over bundled UTP cables.
Several sources of noise affect the signals within the cable. The most influential of these include: Gaussian (thermal) noise, Crosstalk, Echo, Reflection, and External Noise. A short explanation of each of these follows.
Gaussian or thermal noise is generated by the random movement of particles within the conducting medium and is directly related to the temperature of the medium. Gaussian noise follows a non linear spectral power distribution, although for consideration of its effects on the relatively restricted spectrum of broadband signals, Gaussian noise generated within a cable has an essentially linear spectral power distribution. Gaussian noise is also generated within semiconductors and resistors within electronic circuits, where the temperature of the components is significant. Gaussian noise generated within cable pairs is generally of less significance than other types of noise described here.
Crosstalk is caused by the cross coupling of signals between pairs in the cable bundle. Since broadband signals are generally heavily modulated and encoded, these appear as effective noise in the receiving pair. While other factors play a part in the susceptibility of signals in pairs to crosstalk, generally, the effects of crosstalk increase with frequency, cable length and the number of broadband carrying pairs in the cable.
Echo occurs at the interface between the cable pair, in which signals must travel in both directions simultaneously, and the remainder of the circuitry, in which transmit and receive signals are separate, is known, for historical reasons, as a two wire to four wire hybrid. Perfect hybrid action is achieved when all transmit signals from the equipment are directed to the line, all signals received from the line are directed to the receive circuitry and no transmit signals are directed to the receiver. In any hybrid a balancing circuit is required which matches the line impedance. If this balancing network does not exactly match the line impedance, then part of the transmitted signal will be directed to the receiver where, since it interferes with the processing of the wanted receive signal, it appears, essentially, as noise.
Reflection is caused when the impedance of the two wire line and the impedance of the signal receiver do not match. Under these circumstances, part of the signal will reflect from the terminal junction. This has the effect of reducing the received power and causing standing waves in the wire pair where the reflected signal interferes with the incoming signal. Since the signal power has been reduced, it becomes more susceptible to other forms of noise. Simultaneously, the standing waves can cause amplitude and phase changes in the wanted incoming signal which appear as noise in the receiver.
External noise is electromagnetic in nature and may be caused by a number of sources. Examples are welding arcs, amateur radio transmissions, poorly suppressed car ignitions and lightening, although this short list is far from definitive. These sources propagate electromagnetic noise which, if the cable and noise source are in proximity, will be picked up by the cable, as a whole, and can, if of sufficient amplitude, cause the equipment connected to the line to stop working during the duration of the external interference or destroy the equipment. Also, due to the effect know as common mode to differential mode conversion, these external signals, even if not of sufficient amplitude to destroy or paralyze the connected equipment, will appear as noise within the wanted signal and severely degrade the ability of the receiving equipment to process the wanted signal.
Signals in the telephone line pass in both directions simultaneously. This is not so in the remainder of the system, where separate transmit and receive paths are used. These separate paths are required to allow the signal to be amplified, modulated, sent over fibre-optics, microwave radio, etc. all of which can only be carried out on a uni-directional signal.
At the point where the two wire line, where signals pass in both directions, meets the rest of the system, a circuit known as a two to four wire hybrid is used to ensure, as far as possible, that transmitted signals from the equipment are transmitted to the line and that signals received from the line are sent to the receive portion of the equipment.
FIG. 6 shows a hybrid/balance network where the impedance of the balance network balances the impedance of the line. If this balance is exact, then only the wanted signals as shown above will exist. However, if the balance is not exact, then part of the transmitted signal will cross the hybrid circuit and be sent back mixed with the signal received from line. In a DSL system, this partial transmit signal, mixed with the receive signal, will be perceived by the receiver as noise and will detract from the receiver's ability to decode and demodulate the received signal.
At present, in order to overcome this problem, which only exists in DSL systems because of the use of semi symmetrical transmission spectra, a system of echo canceling is used. In this, the digital equivalents of both the transmit and the receive signals are sampled, correlations are sought and any correlations found are cancelled. While that system is able to cancel echoes with present day equipment, working at present day data rates, it suffers from some shortcomings. First, it is complex and expensive, adds to cost and detracts from reliability. Second, it adds delay to the incoming signal, making the synchronization of the overall system more difficult. Thirdly, especially with digital systems such as high speed internet access, it has a tendency to cancel chance correlations with wanted receive data, which may occur from time to time.
As DSL speeds increase in future, the existing echo canceling systems will become less and less able to handle the high-speed data stream. The cost of the system will increase exponentially with data speed and may be a major stumbling block to the development of Very High Speed DSL (V-DSL).
Another problem that occurs at the two to four wire interface is the problem of matching the input of the hybrid to the impedance of the line. This should not be confused with the problem of echo in the hybrid, which is measured as trans-hybrid loss and in which a high figure is desired. Impedance mismatch at the hybrid to line junction gives rise to signals being partially reflected back into the line, reducing the amount of signal energy received from the line. In a system where any additional loss may make the difference between a working DSL link and one that is inoperative, the amount of mismatch can be significant.
It is possible to set the hybrid up to operate into a nominal line impedance, as supplied by the telephone company whose lines are to be used. However, no telephone line has the same impedance as any other telephone line and none of them are likely to conform to the nominal standard. Variations in impedance will be caused by length of line, diameter of the wires, the nature and thickness of the insulation around the wires and even such uncontrollable phenomena as the ambient temperature and the atmospheric pressure.
Under these circumstances, in systems where there exists the opportunity and the time to allow matching to any one line, the balance network will be manually adjusted to provide the maximum trans-hybrid loss and the best impedance match to the line. For systems, such as DSL, where a mass market exists and where there simply is no opportunity to make such adjustments and a nominal impedance and balance has been considered the best that could be hoped for.
Therefore, in present day systems, the use of echo canceling is mandatory and a junction impedance mismatch is the two to four wire junction is accepted as an inevitable and unsolved problem.
From all of the preceding information, it can be seen that the problems which currently beset Asymmetrical DSL have been caused by the presence of excessive far end cross talk at the higher frequencies of the downstream transmission spectrum. This cross talk is mainly caused by the common mode vector. From the presence of this crosstalk, and the piecemeal attempts to solve this problem, arise all the other problems that limit the capacity and competitiveness of DSL systems. However, no attempt seems to have been made to analyze the basic problems and the causes of crosstalk. This patent will undertake such an analysis, and show that the actual problem, once isolated, is amenable to treatment. It will also provide solutions to the problems that are presented. These solutions are in the form of circuits that may be added to existing telecommunication equipment, systems and networks to provide on or more means for greatly increasing the capacity of DSL systems.